Originally Posted by
pdxer
Double-press the sleep/wake button and authenticate with Face ID or double-press the home button and authenticate with Touch ID (one combined step), then hold it near the terminal to pay. Optionally, switch cards before paying.
There's no need to wait for the phone to detect NFC or to unlock it to launch the Wallet app or any other app, although that does work.
Yep, that's what I was referring to when I was saying "pre-authenticating" above. ^